The Eta-Aquariids (ETA#031)

In this article we find 1073 Eta Aquariid meteor orbits obtained by CAMS BeNeLux in 2018 combined with the public CAMS data 2011-2012, that fulfil the low threshold D-criteria. We find a compact radiant with more dispersed orbits at the edges of the activity period. The radiant drift was compared for different threshold classes of D-criteria.

1 Introduction

Exceptional good conditions around the maximum of the η-Aquariids resulted in a rich collection of η-Aquariids at our rather northern latitude. There was not a single night in the entire month of May without multiple station meteors.

Around the maximum of the shower, the number of multiple station events collected by our network was on average about 165 per night in 9 nights of the first decade, with the night 9–10 May as exception. In that night only 20 multiple station events were recorded.

As the month of April already had a lot of clear nights, this was a great opportunity to establish the radiant drift of this shower using the data that CAMS BeNeLux collected in 2018 together with the data that can already be found in the CAMS database (Jenniskens et al., 2016).

2 Available data

The CAMS database (2010 – March 2013) and data from CAMS BeNeLux were selected for the analysis from April 15 to June 1. In total, 17223 orbits were available during this period.

The orbital elements from Jenniskens et al. (2016) were used as reference orbit for the η-Aquariids.

Next, the discrimination criterion of Drummond (1981) determined which orbits could be considered as η-Aquariids. In total we found 1073 orbits with DD < 0.105, and another 504 orbits within the following intervals:

320° < αg < 360°;

–10° < δg < +10°;

55 km/s < vg< 75 km/s.

The orbits of the η-Aquariids were divided into five D-criterion classes:

low (0.08 < DD < 0.105)

medium (0.06 < DD < 0.08)

medium high (0.04 < DD < 0.06)

high (0.02 < DD < 0.04)

very high (DD < 0.02)

Figure 1 – Plot of the inclination i versus length of perihelion Π for the 1073 η-Aquariids and 503 non η-Aquariids.

If we look at the plot of Π versus i of these 1073 orbits (Figure 1), divided into these five classes, we see a concentration of orbits in the center with DD < 0.02 with an increase of orbits outwards with lower D-criterion classes. A gradual smearing out of the shower orbits with the sporadic orbits seem to occur.

Those meteoroids with higher values of the D-criterion were probably long ago released by the comet, and by planetary perturbations, and for the smaller dust particles effects of the solar wind, drifting further away from the parent body, resulting in increasing differences in orbital elements. This process continues until the particles can no longer be distinguished from the random sporadic background.

For a meteor stream with low inclinations this will probably occur fairly quickly, but for the ETAs the speed is quite distinctive due to the retrograde orbit, so that visual observers can still identify them, even relatively far from the maximum activity.

Table 1 – Number of η-Aquariids per DD-class, per degree of solar longitude.

λʘ

±0.5°

<0.105

>0.08

<0.08

>0.06

<0.06

>0.04

<0.04

>0.02

<0.02

31.5

1

0

0

0

0

32.5

0

0

0

0

0

33.5

0

0

0

0

0

34.5

0

0

0

0

0

35.5

1

0

0

0

0

36.5

1

1

0

0

0

37.5

1

2

3

0

0

38.5

0

3

5

0

0

39.5

5

6

7

2

0

40.5

4

10

9

7

0

41.5

6

8

19

15

0

42.5

7

11

20

22

2

43.5

7

12

17

37

11

44.5

8

11

10

20

16

45.5

16

20

22

50

39

46.5

15

13

24

47

59

47.5

12

11

15

20

24

48.5

12

6

12

13

10

49.5

2

12

18

33

5

50.5

8

3

19

31

0

51.5

10

11

11

16

0

52.5

3

6

12

4

0

53.5

2

1

9

2

0

54.5

1

8

10

0

0

55.5

5

7

9

0

0

56.5

0

2

1

0

0

57.5

6

4

1

0

0

58.5

3

7

0

0

0

59.5

0

4

0

0

0

60.5

7

2

0

0

0

61.5

0

0

0

0

0

62.5

3

1

0

0

0

63.5

4

0

0

0

0

64.5

1

0

0

0

0

65.5

1

0

0

0

0

66.5

0

0

0

0

0

67.5

0

0

0

0

0

68.5

1

0

0

0

0

69.5

0

0

0

0

0

3 The radiant drift

For the η-Aquariid orbits we found the following numbers in each DD -class:

DD < 0.020 ; 165 orbits

DD < 0.040 ; 485 orbits

DD < 0.060 ; 737 orbits

DD < 0.080 ; 920 orbits

DD < 0.105 ; 1073 orbits

For each of the η-Aquariids in these classes, both the right ascension and the declination were plotted against the solar longitude. Subsequently, by means of the least-squares method, a best linear fit was determined through the points with corresponding standard deviation (Doom and Bouma, 2018).

The differences between all five classes are small for the drift in right ascension as for the drift in declination. All the values are mentioned in Table 2.

Table 2 – Radiant drift with ± σ for the η-Aquariids obtained from the orbits for each threshold level of the D-criteria compared with a reference from literature.

Threshold/source

ETA – 031

Δα / λʘ

Δδ / λʘ

Low

0.72 ± 0.01

+0.33 ± 0.01

Medium low

0.71 ± 0.01

+0.32 ± 0.01

Medium high

0.71 ± 0.01

+0.32 ± 0.01

High

0.72 ± 0.01

+0.32 ± 0.01

Very high

0.76 ± 0.02

+0.34 ± 0.02

Jenniskens et al. (2016)

0.92

+0.37

The radiant positions of the η-Aquariids orbits in equatorial coordinates are plotted in Figure 2 together with the linear regression line for the low similarity class (blue line) and for the very high similarity class (dotted line).

Figure 2 – The radiant positions of the η-Aquariids orbits in equatorial coordinates. The colors indicate the similarity class like in Figure 1. The linear regression for the radiant drift for all orbits with DD < 0.105 is plotted in blue, for DD < 0.02 as a dotted line.

The radiant drift corrected positions for all η-Aquariids are plotted in Figure 3. Also, here we see the concentration of orbits with DD< 0.02 as mentioned above, in the center, with more orbits of the lower similarity classes towards the edges.

If we select all 1073 η-Aquariids plotted in Figure 1, adding a color gradient to show the variation in geocentric velocity, we see a remarkable concentration of slower η-Aquariids at left and another distinct concentration with faster η-Aquariids at right (Figure 4). We don’t have yet an explanation for these slower and faster η-Aquariids concentrations. This point may need further investigation.

Figure 4 – The inclination i against the length of the perihelion Π with a color gradient to show the spreading on the geocentric velocity.

4 Conclusion

We find a radiant drift Δδ / λʘ of 0.34° in good agreement with the values obtain by Jenniskens et al. (2016). For the drift in Right Ascension we find a slightly smaller value than Jenniskens et al., Δα / λʘ = 0.76°.

Far ahead of and far behind the maximum activity we find barely any more orbits with a similarity value of DD < 0.04.

These are probably particles that got further away from the parent body due to planetary perturbations, with as a consequence increasingly deviating orbital elements.

Two remarkable concentrations of slower and faster η-Aquariids need further investigation to find an explanation.

Acknowledgment

The authors thank Reinder Bouma for his comments on this article and Denis Vida for providing a tool to plot a color gradient.